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When we change the slider-arm position, we change the receiver’s sensitivity to the frequency of the station we were listening to. At the same time, however, the moving slider arm makes the receiver sensitive to other frequencies. And we can now pick up stations broadcasting on those frequencies . . . provided, of course, these stations are in the area and transmit a fairly strong signal.

Things You Need

Tuning Coil: spool of #16 magnet wire. Core: wood dowel 1” diameter by 5”. Slide Arm: stiff piece of metal about 5” by 3/8”. Base: wood, 8” by 8” by 3/4”. Capacitor: mica capacitor, 0.002 mfd. Crystal Detector: germanium diode. IN34A. Antenna: wire, 50 to 100’ long (use whatever kind you have on hand, bare or insulated . . . if bare, you'll need a glass insulator at each end of the antenna). Headphones: a high-impedance pair of 2000 ohms (the common transistor-radio earphone may not work to well, if at all). Minor Items: 4 Fahnestock spring clips, 5 screws, 2 washers, 6 small nails, 2 tin-can strips 1” by 1-1/2”, hook-up wire.

How to Build It

TUNING COIL AND CORE. If you can’t find any 1” wood dowel for the core, there are other things that will work as well. . . maybe even better. A piece of 1” outer diameter rubber hose would be excellent. So would a stiff plastic tube or rod. Or, take a piece of “one by two” wood, which actually measures 3/4” by 1-1/2”, and saw it lengthwise. This will give you a piece 3/4” by 3/4”.

After selecting your core and cutting it to the 5” length, measure off l/2” from each end. Then drill (or pierce) a small hole through the core at both marks. These holes will keep the, magnet wire in place. Thread about 4” of wire into one of the holes, and begin winding the coil. Keep the turns close to one another. It will take about 70 turns to reach the other hole. When you reach it, thread the wire through and cut off all but 4”.